The present invention relates in general to measuring and controlling at least one characteristic of a web of sheet material as the web is being manufactured and, more particularly, to methods and apparatus for measuring and controlling a web of sheet material as it is being manufactured by means of a first sensor, preferably a stationary sensor, to maintain the uniformity of the web and a second sensor, also preferably a stationary sensor, to maintain the absolute value of at least one characteristic of the web. While the present invention is generally applicable to measuring and controlling the manufacture of a number of different types of web material, it will be described herein with reference to webs of paper for which it is particularly applicable and is initially being applied.
A variety of sensors have been utilized to measure and control the manufacture of sheet material which is manufactured as continuous webs moving at high speeds in the manufacturing process. The web material can be, for example, metal, plastic or, for the description of the present application, paper.
The most common and currently popular form of paper web sensor is generally moved across the web of paper in what is referred to as a cross direction (CD) scan of the web. As the sensor, for example a beta gauge basis weight sensor, is scanned across a paper web in the cross direction, the paper is moving rapidly through the paper making machine. Accordingly, the sensor detects the basis weight of a zig-zag pattern on the web in the direction of web movement or machine direction (MD). With scan speeds ranging from 100 to 400 millimeters (mm) per second, scanner instrument measuring zones ranging from 10 to 30 mm and web speeds ranging from 3 to 30 meters per second, less than 1% of the web is measured using such scanned gauges.
Separation of web property variability into cross direction and machine direction causes and effects is difficult and time consuming due to the limited machine direction measurement frequency resulting from the zig-zag scan pattern and the limited amount of the web which is actually sensed. In addition, typically five to ten CD scans must be completed before sensor results can be determined, possibly a time period of up to 30 or more minutes. Added to the delay necessitated by multiple scans, sensor scanners are normally located close to the take-up reel for the machine which results in additional delay for the paper to leave the headbox, traverse the machine and arrive at the sensor. This web travel delay is added to the time required before the effects of any corrective measures can begin to be evaluated.
In an attempt to speed up web measurement and control, a stationary optical sensor extending continuously across a paper web is disclosed in U.S. Pat. No. 5,071,514. As disclosed in the '514 patent, the stationary optical sensor must be calibrated for controlling the machine making the paper web. Calibration may be performed by a closely associated scanning optical sensor which senses discrete regions of the web as it is scanned across the web.
The noted stationary and scanning optical sensors are positioned at the wet end of the paper making machine and are supplemented by scanning sensors located at the dry end of the paper making machine. Data from the scanning sensors at the dry end of the paper making machine may be correlated with the stationary optical sensor for ultimate control of the paper making machine. Unfortunately, in the system of the '514 patent, the calibration performed by the wet end scanning optical sensor is not entirely accurate such that calibration effectively must be delayed until correlation of the dry end scanning sensors with the stationary optical sensor located at the wet end of the machine.
A second stationary optical sensor is disclosed in U.S. Pat. No. 4,950,911 for performing inspection of a sheet of material to detect flaws which occur within the sheet of material. In this system, threshold levels are adapted on a pixel by pixel basis in response to raw data coming in from the stationary optical sensor. Event signals are generated for pixel signals which pass through the threshold levels with the event signals being used to identify flaws in the sheet of material which is being inspected. While the system of the '911 patent is very effective for detecting flaws in sheets of material, the system is limited in terms of speed and therefor data collection rates. Further, no provision is made for controlling the machine making the sheet material in response to data generated by the inspection system.
There is thus a need for an improved arrangement for monitoring webs of sheet material for accurate determination of sheet characteristics and use of those characteristics to rapidly control the machine making the webs. Preferably, the monitoring arrangement will be able to collect data for statistical analysis of the material and the process making the material. Also the monitoring arrangement would monitor the sheet material near the initial processing of the material, i.e. near the wet end of a paper making machine, and be able to independently control the uniformity of the webs of sheet material. If characteristics of the sheet material are to be absolutely controlled, they would be controlled from a second sensor, preferably a stationary sensor, monitoring the web near the final processing of the sheet material.